Haptic interfaces permit a user to experience a sense of touch in a virtual or haptic environment. Such interfaces are finding acceptance in virtual reality games and in performing tasks that are virtually imaged. One area which uses haptic interfaces to help a user perform a task is computer aided surgery.
In computer aided surgery, a haptic interface can be used to provide haptic guidance to a surgeon. For example, a surgical instrument, such as a cutting tool, can be coupled to a haptic interface. The haptic interface may be, for example, part of a robotic device, such as a robotic arm. As the surgeon moves the surgical instrument in real space (e.g., to cut bone or other anatomy), constraints may be imposed on the surgeon through the haptic interface that limit his ability to manipulate the surgical instrument. For example, the surgeon's ability to manipulate the surgical instrument may be constrained so that the surgeon can only move the surgical instrument within a defined cutting region. The constraints may be based, for example, upon a desired relationship between the surgical instrument and the defined cutting region in real space. This real space relationship can be represented in a virtual environment as a relationship between a virtual instrument and a haptic object in virtual space, where the virtual instrument corresponds to the physical surgical instrument and the haptic object corresponds to the defined cutting region. In operation, the surgeon manipulates the surgical instrument while it is attached to the end of the haptic interface. Constraint feedback is provided to the surgeon through the haptic interface, which imposes a force on the surgeon sufficient to maintain the desired relationship between the virtual instrument and the haptic object.
For example, the haptic object may be a virtual protective boundary for an anatomic structure or a shape that is to be cut into bony anatomy. The virtual boundary is registered (or correlated) to the anatomy of a patient, and the virtual instrument is registered (or correlated) to the actual surgical instrument. To enable the surgeon to interact with the virtual environment via the haptic interface, a haptic rendering algorithm is employed. Haptic rendering is the process of computing and applying forces in response to user interactions with virtual objects. Using the haptic rendering algorithm, the haptic interface may be configured so that the force experienced by the surgeon increases as the virtual instrument approaches the virtual boundary. This increasing force provides a warning to the surgeon that he is near a forbidden region of the workspace (e.g., an anatomic structure of interest or other boundary) and therefore should proceed with caution in order to prevent unwanted penetration into and damage to the structure (for example, preventing a drill bit from entering too deeply into the bone). If the surgeon tries to force the instrument beyond the virtual boundary, the haptic interface provides an increasing force to prevent such motion. In this manner, the virtual boundary functions as a haptic stop to maintain the surgical instrument within a desired region of the workspace.
Preventing movement using a haptic stop, however, is typically accomplished with an admittance or impedance based system. An admittance device senses forces exerted by a user and responds by changing the position of the device (e.g., the position of the surgical instrument). Although admittance devices can provide stiff boundaries, they require force sensors and generally feel heavy to the user as the user moves the device through free space. In contrast, an impedance device senses a position of the device (e.g., a position of the surgical instrument) and responds by applying forces to the device by applying limited power to actuators of a backdrivable haptic interface system. Impedance devices generally feel relatively light to the user when moved through free space and are preferable for certain applications where the users wants to have a relatively light motion and to feel interaction forces with real objects when moving in free space. The output force of the actuators, however, is finite, and impedance devices are not able to generate boundaries as stiff as those generated by admittance devices. As a result, it may be possible for the surgeon to overcome the constraints imposed by the actuators and force the surgical instrument past the virtual boundary or haptic stop. This would result in unplanned damage to the tissue being operated upon.
What is needed is a haptic device having an adjustable positive stop that can provide haptic constraint forces sufficient to prevent erroneous movements by a user of the haptic device while still permitting the user to experience flexibility of motion during operation of the haptic device.